Far more than at Weenie Roast, that summertime bacchanal in Irvine where partying matters as much as performances, KROQ’s yearly weekend of Almost Acoustic Christmas shows at Gibson Amphitheatre checks the pulse of modern rock.

Yet all along — and arguably more so these days — AAC has served as a proving ground. It’s still the most coveted (and toughest) one this side of Coachella. Doesn’t matter what sort of act spins into view on this rotating stage — rising or struggling newcomers, old hands looking to reintroduce themselves, established titans extending their reign. They all face fickle, largely young Southern California crowds whose rabid anticipation can rapidly dissipate into boredom if they aren’t grabbed from the get-go and kept rapt. Here, you either bring it, or you leave knowing several thousand teenagers are about to trash you endlessly.

The good news out of Saturday night’s more mosh-tastic first half is that by and large everyone measured up, although given how many people trickled away by the end of Billy Corgan & Co.’s 11-song closing turn, I suspect the jury may still be out regarding the Pumpkin King soldiering on with new, considerably less powerful players.

You can understand the reluctance two decades after the real Chicago thing debuted. At least when he closed up shop the last time, with a pair of shows two years ago at this same venue, his attempt to pull a Pretenders still had Jimmy Chamberlin on drums, plus all of No Doubt’s auxiliary filling in the blanks. Now Corgan is backed by Mike Byrne, a ham-fisted drummer of little finesse, and flanked by what seem like junior apprentices: guitarist Jeff Schroeder, merely a pale imitation of his boss, and bassist Nicole Fiorentino, functioning as eye candy more than anchor (yet she doesn’t have half as much mystique as woebegone D’Arcy Wretzky.)

It left me perplexed, though their chunky stew can still coalesce into some hearty rocking when Corgan bothers to light a fire under it. Yet, apart from his indelible melodies and occasional Fender flourishes, this doesn’t even sound like the Pumpkins anymore, which I bet is his point; he’s rarely seemed in a better mood on stage, so maybe he’s finally achieving some Zen autonomy at last with this lineup.

I’m trying to reserve judgment, see where this Pumpkins 3.0 (4.0? higher?) might lead. But so far it feels too much like a cheat: “Bullet with Butterfly Wings” was recklessly rushed (and thus less impactful), “Tonight, Tonight” had no sense of whirlwind romantic urgency, and “Today” was borderline amateurish. And still I wish they’d selected more songs like those.

That’s my other beef: when initially presenting a newly carved Pumpkins, stuff its guts with as many Big Ones as possible. I know Corgan insists on chasing his idiosyncratic muse — it isn’t his style to leave off “Tarantula” and “Stand Inside Your Love” to make room for more popular fare like “Disarm” and “Cherub Rock” and “Zero,” all missing here. But this time that move was a mistake, and he failed to maintain audience interest. I wanted to leave after his rendition of Fleetwood Mac’s “Landslide,” too; it’s the only thing I’ll remember from this set this time next year.

Corgan should have taken some tips from Mike Ness. In a set almost as long – I would have dumped the forgettable A Day to Remember and the valiant Switchfoot (decent on a blast through the Beasties’ “Sabotage”) to allow three or four longer sets later on – the still-vital stalwart that spearheads Social Distortion illustrated just how expertly a band can introduce new ones in a rundown that also touches on key staples and a complementary gem or two.

Emerging out of the shadows, dapper in a blue gangster/pachuco suit and hat, the O.C. punk veteran was in sharp, sweaty form from the start and selected wisely straight through, serving up an ideal sampler to whet appetites for January’s bigger onslaught. That’s when the group’s great, ’70s-fueled new album, Hard Times and Nursery Rhymes, arrives (Jan. 18), followed by a string of dates at the Hollywood Palladium (Jan. 27-29) to be preceded by a return to House of Blues Anaheim (Jan. 12).

The older choices tell the tale of the new disc: material from Mommy’s Little Monster was passed over in favor of revisiting rarities from the ’90s like “Through These Eyes,” “So Far Away” and an almost Tom Waitsian take on “Cold Feelings,” for these “Hard Times” tracks gather up the heavy force and emotional heft of such songs, then add in a welcome dose of Stones blooze and New York Dolls swagger. Offering only “Ball and Chain” and “Story of My Life” to hook in casual KROQ fans amid this first glimpse could have gone really wrong; instead, it just kept gaining strength. Next month’s full-length performances could be revelations.

As for the rest of the lineup:

My Chemical Romance probably should have headlined, in what would have been a major moment from an arena-level band just getting back on the road behind another musical advancement, the highly appealing new album Danger Days: The True Lives of the Fabulous Killjoys. Whereas the new Pumpkins are still finding their footing, MCR has bounced back into shape in lean, fighting form, much the same as AFI did here last year.

That said, I’m not complaining that, before Gerard Way turns on the theatrics and the performances get elaborate again, fans got to catch this fiery warm-up, a shot mixed of current singles, Black Parade highlights and some of the best cuts from before then.

And though Bad Religion, Jimmy Eat World and Cake sound nothing alike, each approached its 40-minute allotment similarly, covering radio-hit bases while tossing in new ones: worthy freshness from the punk mainstay and better than expected from Jimmy but less interesting tunes from John McCrea and his Sacramento gang.

Greg Graffin’s songwriting continues to improve with age; a song like “Cyanide,” off this year’s 15th Bad Religion album The Dissent of Man, deftly blends the personal with the political and sits well alongside the dynamics of “Sorrow” and “Los Angeles Is Burning.” Jimmy’s new stuff from “Invented,” while not as engaging (I’m never as swept-up by their hooks), undeniably stirs the same passions among fans as the Arizona band’s strongest work — and all of it, past and present, was convincingly performed.

But Cake’s freshly penned bits seemed rote: “I’m so sick of you, so sick of me, I don’t want to be with you” may make for an amusing call-and-response but it’s lyrically beneath McCrea’s skewed sensibilities and knack for zingers. That doesn’t exactly have me excited about Showroom of Compassion, their first album in seven years, due Jan. 11.

Thankfully, such moments were just sprinkles on an otherwise filling Cake, still with Vince DiFiore on trumpet, Gabe Nelson on bass and Xan McCurdy on guitar. Stopping by all their expected signposts — “Short Skirt/Long Jacket” to open, “Frank Sinatra” and “Never There” in the middle, “The Distance” to end — plus their (unironic?) blast through Black Sabbath’s “War Pigs” has me eager for a stretch of Southern California shows. For if ever a band needed to be seen in its own environment, it’s Cake.

Oh, and Anberlin: when did they start dressing like Maroon 5? I suppose that makes them more camera-ready, and they are competent and improved — but they still bore me to tears.

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